This invention relates generally to frequency doubler circuits and, more particularly, to frequency doubler circuits capable of operation at relatively low frequencies and capable of handling square-wave signals. Frequency doubling is a commonly required function in many applications of electronic circuitry. A technique commonly used for frequency doubling is to phase-shift an input signal by 90° (90 degrees) and then to apply the input signal and the phase-shifted signal to an exclusive OR (XOR) gate. The XOR gate generates an output signal at twice the frequency of the original input signal. Unfortunately, it is difficult to achieve a 90° phase shift in an integrated circuit for low frequencies, or for a square-wave input signal.
A conventional frequency doubling circuit has problems in handling either square waves or low-frequency signals, and when constrained to draw a constant current from its power supply. Constant current consumption is, however, a requirement in many integrated circuit applications. A convenient and commonly used circuit for achieving a 90° phase shift in input signals covering a broad band of frequencies is a resistor-capacitor/capacitor-resistor (RC-CR) filter, which is depicted in FIG. 1. In this circuit, an input signal applied to an input terminal 10 follows two parallel paths to two output terminals 12 and 14. One of these parallel paths includes a series capacitor 16 and a shunt resistor 17, which together form a high-pass filter. The other parallel path includes a series resistor 18 and a shunt capacitor 19, which together form a low-pass filter. This circuit is commonly used for generating two output signals with a 90° phase difference, from a sine wave input signal. Unfortunately, for a square-wave input, although the output will be a phase-shifted square wave output at terminal 12 from the high-pass side of the filter, the output at terminal 14 from the low-pass side of the filter is a phase-shifted triangle wave. Moreover, the zero crossings of the phase-shifted square wave and triangle waves will not be exactly 90° out of phase. Because of these discrepancies, when this filter circuit is coupled to an XOR gate, the resultant output is not always the desired double-frequency signal.
Accordingly, there is a need for a frequency doubler circuit that will function well for low-frequency and square-wave inputs. The present invention is directed to this end.